1. Field of the Invention
This invention relates generally to electronic image display systems, and more particularly to direct view and projection color image display systems with an improved cathode-ray tube construction.
2. Description of Prior Art
Conventional full color displays are created through the use of three phosphors which are deposited on the faceplate of a cathode-ray tube. There is one phosphor for each of the primary colors which are mixed to produce full color. Each of the phosphors is energized by an independent electron gun. The phosphors are positioned on the inner surface of the faceplate of the cathode-ray tube as either a multitude of phosphor dot triplets, or as an array of narrow vertical stripes. The electron beam from each of three electron guns, one for each of the primary colors, passes through small openings in a shadow-mask before reaching and energizing the respective phosphors on the faceplate of the cathode-ray tube. The mask is positioned so that each electron beam strikes only its separate phosphor position (and respective color) while all remaining energy is absorbed by the mask. The mask produces a shadow over most of the phosphors allowing only the desired phosphor to be energized.
In conventional cathode-ray tubes, the electron beams from the electron guns are deflected both horizontally and vertically by either electrostatic or magnetic fields to produce an image. One complete scan produces a frame on the faceplate of the cathode-ray tube which is visible as an image to the viewer. The image is refreshed at such a rate as to produce a non-flickering image to the viewer.
Using a shadow mask technique limits the resolution of the cathode-ray tube due to the coarseness of the mask; there is a limit on how finely the mask can be made. Another drawback of the shadow mask relates to image brightness; up to 80% of the electron current is absorbed by the mask and is unavailable to energize the phosphor.
In addition to the resolution and brightness problems, conventional cathode-ray tubes are bulky; the tube depth is generally greater than the diagonal of the faceplate. The ultimate size of a direct-view cathode-ray tube is physically limited by the nature of a vacuum tube; larger tubes require thicker glass to accommodate faceplate strength resulting in heavier and heavier tubes.
It would be desirable to provide a direct view full color display based on a cathode-ray tube that is lightweight and compact, with higher resolution, and which has a flatter profile.
With respect to projection displays, heretofore such displays have been created by using three individual cathode-ray tubes, each with a different colored phosphor. Three separate optical systems are employed to combine the three individual images from the cathode-ray tubes into one image, typically being projected upon a diffusion or lenticular type screen.
One major problem with conventional projection display systems, however, is that their image resolution is limited by the practical size of projection tubes. A complex optical system is required to converge the three images from the three separate cathode-ray tubes onto the screen. Moreover, typical projection display systems with three cathode-ray tubes and three independent optical systems are costly, heavy and difficult to manufacture. In addition, since each cathode-ray tube is a separate full image display, a tradeoff must be made in order to provide cathode-ray tubes which are large enough to accomplish required image resolution and brightness, but not so large as to make the display system impractical.